The invention relates to a process for the preparation of a water resistant fuel agglomerate. It is directed also to the composition of matter employed in this process.
By the expression "fuel agglomerates", is meant any physical presentation of finely divided fuel materials which are easily handled and useful for domestic or industrial purposes. By way of example may be mentioned nuts, briquets and pellets.
The finally divided fuel materials concerned by the present invention may be any substances rich in carbon, like for example coal fines or dusts, wood fines, coal coke fines, fines of petroleum coke or a mixture of these products. These materials and in particular coal fines and dusts are produced in large amounts by modern methods of extraction and washing, particularly of coal.
Among the value-enhancing uses of these materials, may be mentioned more particularly their use in the form of fuel agglomerates.
Different techniques of agglomeration of these fines or dusts, generally employing additives or binders suitable for ensuring sufficient cohesion, have already been proposed.
Among these additives or binders, the most employed currently are tar from coal, from wood or from petroleum, lignosulfonates, clays, polysaccharides among which in particular are starches and starch derivatives.
The most used of these binders is incontestably coal tar, but the exigencies as regards protection of the environment becoming more and more strict, its use today shows a certain recession.
In fact, its use makes it necessary to submit the agglomerates so obtained to a heat treatment or fume reduction treatment in order to lower the concentration of phenolic compounds. Now this treatment results in a non negligible atmospheric pollution. Moreover, the fume reduction treatment not being complete, the combustion of these agglomerates at the time of their use causes a release of smoke noxious to man.
These drawbacks have led certain countries to forbid their use.
The drawbacks inherent in the use of tar reoccur during the use of bitumen as binder.
To overcome these drawbacks, it has been proposed to resort, as binder, to lignosulfonates, in particular that of ammonium.
The scientific literature relating to the use of these products is extremely abundant and there may be mentioned by way of example patents SU 983.147, SU 1.010.146 and SU 1.137.103, patents EP O 097 486 and DE 3.277.395 or again patents DD 224.331 and U.S. Pat. No. 4,666,522.
It happens that the technique of agglomeration with lignosulfonates is complex and its performance requires considerable mastery. In particular, it is necessary to dry the fines to a precise moisture content so that the lignosulfonates-fines mixture may be agglomerated, an excess or a lack of moisture making this operation impossible.
On the other hand, in the heat treatment, to polymerise the lignosulfonates and thus to confer on the agglomerates a good resistance to water, there is produced a disengagement of noxious fumes rich in sulfuric acid, a non negligible cause of atmospheric pollution.
It has been proposed to resolve this pollution problem by applying different arrangements to the installations concerned, and particularly by providing smoke condensing devices. But such devices have only had the result of displacing the pollution problem to a corrosion problem which it is known to have been extremely difficult to control especially when it is a matter of treating condensates rich in sulfuric acid, even by employing special steel as constituent material of the agglomeration installations. In any event and whatever the solutions envisaged, the drawbacks associated with the use of lignosulfonates make it a laborious technique.
Moreover, the agglomerates manufactured according to this technique show the drawback of generating, during their combustion, sulfurous residues which are again found particularly in the smoke.
Processes have been proposed not showing the above mentioned drawbacks associated with pitch, with tar and with the lignosulfonates, in which the binders are replaced by clays particularly by bentonite (U.S. Pat. No. 4,025,596 and DE 1.671.365). However, the agglomerates obtained according to these techniques do not all show the required physical properties; in particular, their mechanical strength is insufficient and their behaviour with water mediocre. Consequently these processes have not been developed in practice.
It has also been proposed to resort, as binder, to starch which, used along or in a mixture with other binders, as taught, for example, by patents U.S. Pat. No. 3,726,652 and DE 3,227,395 or again patent EP 0 097 486, shows numerous advantages.
A comparative study on pelletisation done in 1982 at the University of Berkeley (thesis of K. V. S. SASTRY and D. W. FUERSTENAU), showed that, with respect to an asphalt emulsion, or to bentonite, starch led to better results on the scores:
of resistance to mechanical compression, PA1 of abrasion resistance, PA1 of impact resistance.
Moreover, starch may be used without limitation in industrial plants initially designed in the use of tar or bitumen which are the binders most used currently, its employment hence not requiring an additional investment; in addition, maintenance of the plants is reduced.
Finally, combustion of agglomerates bound with starch does not generate a toxic and/or polluting smoke.
However, and this constitutes a major drawback, agglomerates based on starch, just as those based on bentonite, show a very distinct sensitivity to water, making their storage in the open air impossible.
It has been proposed, to overcome this drawback, to associate starch with tar, with asphalt or with bitumen or again to insolubilize the starch with resins of the urea-formol, phenol-formol, melamine-formol, ketone-formol type or their admixture.
None of the solutions is satisfactory since all rest on the problem of release of toxic and polluting smoke during the combustion of the agglomerates so obtained.
It has also been proposed (see U.S. Pat. No. 1,507,673), to render these fuel agglomerates based on carbohydrates water resistant, to incorporate therein a strong acid in proportions which are not negligible, among which are in particular phosphoric acid, and to treat said agglomerates at a temperature comprised at a temperature comprised between 200.degree. and 540.degree. C.
This solution is not satisfactory since, during the treatment, there remains, as for the lignosulfonates, the problem of release of corrosive fumes. In addition, the handling of strong acid is always a delicate and hence constraining operation.
It has also been proposed to coat the agglomerates with a water-repellent sheet or film obtained by the application of an emulsified wax. Although original, such a solution is laborious through the amount of wax employed and the protection against moisture so conferred on the agglomerates may be altered if these agglomerates undergo, in the course of their transportation, impacts resulting in a deterioration of the protective sheet.
It has lastly been proposed (see patent EP 89 400071) to prepare water resistant fuel agglomerates comprising, distributed within their constituent mass, on the one hand, a carbohydrate as binder and, on the other hand, an organosilicic agent as a water repellent agent.
Such agglomerates, although having suitable resistance to bad weather have the drawback of having a relatively fragile surface condition when they are moist. Such sensitivity is manifested by a well known deterioration of their skin during their handling, thus generating dust in non negligible amount.
Consequently, none of the existing processes enable the production, under economically and ecologically acceptable conditions, of fuel agglomerates having simultaneously satisfactory mechanical properties and behaviour to water.